Heavy vehicles require multi-stage transmission assemblies which are equipped with starting friction clutches and means for interrupting the power transmission when speed ratios are changed. In order to increase speed it was necessary in recent years to enhance the specific performance for such vehicles so that the load on the friction clutches in use, particularly in starting on upward gradients, was at the limit so that the friction discs carrying the linings had to be replaced after comparatively short runs. The replacement of friction clutches by hydrodynamic devices causes a loss of efficiency. In addition, this involves the disadvantage of steady additional heat generation in the transmission housing. This is why it has been endeavoured to equip starting clutches for heavy vehicles with liquid cooling. However, since both components to be coupled rotate, there is the difficult task of providing a rotating supply of coolant.
Again, transmission assemblies of the type designated as powershift or clutch transmission assemblies equipped with common hydraulically actuated clutches which are engaged and disengaged respectively to change the speed ratios, require a starting clutch irrespective of whether it is a countershaft or planetary transmission. An additional difficulty in shifting clutch transmission assemblies of this type resides in the fact that, after disengaging one clutch, the clutch of the next speed ratio must engage as promptly as possible so that the slipping of the clutch does not cause too much heat and too much abrasion on the friction linings. In addition, the power transmission must be interrupted as little as possible on the one hand while no shock must be caused during shifting on the other. The shifting process thus constitutes a compromise of which the optimum is difficult to achieve by controlling the pressure in the hydraulic system so that shifting shocks frequently occur when speed ratios are shifted in such clutch transmission.
The present invention has for its object to provide a novel device for starting which is free from the above disadvantages and replaces presently know starting clutches between the engine and the transmission assembly. The invention is furthermore disigned to provide an actuating system which not only controls the starting unit but also avoids the above-mentioned shifting shocks.